Listen for stunning songs of wood thrush

In the spring while in the woods, listen for a bird song that seems too hauntingly beautiful to be of this world. It sounds as if the bird is singing a simple "e-o-lay" phrase, but it's different from most bird songs because it seems to reverberate and echo throughout the trees.

What you are hearing is a wood thrush — Hylocichla mustelina — a neotropical migrant belonging to the same family as robins, hermit thrushes and bluebirds. Like other thrushes, the wood thrush has a syrinx (voice box) that allows it to sing complicated songs, harmonizing with itself by singing two notes at once.

The wood thrush nests in a variety of forested areas. The male searches for a site that he thinks will be a good place to raise the young. The lower branches of a young tree or shrub in a shady area will usually meet his specifications. Hoping to lure his lady friend, he will place nesting materials nearby.

If she thinks he's made a good choice, she will build the nest herself, to her specifications. She carefully selects materials that will blend in with the surroundings and her markings. She loves cinnamon-colored materials because they match her back and the branches of many small trees. She weaves in lighter materials to match dappling in the forest created by the combination of light and shadow. The dappling also will match the spotting on her breast.

Wood thrushes usually raise two broods per year. They will not reuse the same nest but will nest in the same vicinity if the first brood was successful. If unsuccessful, they will abandon the area and move to a better neighborhood.

Wood thrushes, although still common, are very sensitive to forest fragmentation and acid rain. Their numbers have declined more than 40 percent during the past 40 years.

Parasitism of nests by cowbirds is one factor in their decline. Cowbirds prefer to slip their eggs into other birds' nests found along the forest edge, a preferred location for many wood thrushes. The cowbird young outcompete some, if not most, of the wood thrush's young. Development of forested areas creates more forest edge, which results in more cowbirds.

Acid rain is caused by sulfur dioxide and nitrogen oxides combining with other compounds found in the atmosphere to form acidic precipitation that falls to the Earth. Although sulfur dioxide is emitted by volcanoes and other sources, and nitrogen oxides are produced by lightning strikes, the main source of these compounds is the combustion of fossil fuels, especially from coal-fired power plants.

This acid rain kills off the invertebrates in the leaf litter on which the wood thrushes depend. It is especially hard on land snails, which are an important source of calcium for egg-laying. A shortage of calcium results in thin-shelled, brittle eggs that can easily break.

Online digital recordings totally fail to capture the essence of this bird's wonderful song. Something is lost — the resonance, the echoing, the otherworldly attributes that define it. Protection of large swaths of unfragmented forests and reduction in fossil fuel emissions will help assure the survival of wood thrushes. Hopefully, future generations will be able to hear, in person, one of the most beautiful bird songs sung in America.

This is the opinion of Jan Bergstrom, whose column appears the first Sunday of each month in Our Woods & Waters. She welcomes questions and comments at janbergstrom2@gmail.com.